Forward osmosis process

ABSTRACT

A forward osmosis apparatus is improved. A forward osmosis apparatus, comprising a diluting means for bringing a feed solution and a draw solution comprising a cation source and an anion source in an ionized state into contact through a semi-permeable membrane and diluting the draw solution with water separated from the feed solution by means of the semi-permeable membrane;
         a separating means for separating the draw solution that has been diluted by the diluting means into the cation source and anion source and into water; and   a dissolving means, returning the cation source and the anion source that have been separated by the separating means to, and dissolving the cation source and anion source in, the draw solution that has been diluted;   wherein the molecular weight of the cation source in an uncharged state is 31 or greater and the Henry&#39;s law constant of each of the anion source and cation source is 1.0×10 4  (Pa/mol·fraction) or greater in a standard state.

The present application is a Divisional Application of U.S. applicationSer. No. 13/827,047, filed Mar. 14, 2013, which is a ContinuationApplication of PCT/JP2011/072261, filed Sep. 28, 2011 and claimspriority under 35 U.S.C. § 119 of Japanese Patent Application No.218911/2010, filed Sep. 29, 2010, the disclosures of which areincorporated herein by reference.

The present invention relates to a forward osmosis process (FO process)and to a forward osmosis apparatus (FO apparatus) capable of conductingseparation and/or concentration by this FO process.

BACKGROUND ART

Water shortages have become a serious problem in arid regions of theworld and in regions of high population density. Accordingly, there is aneed for desalination technology that removes salt from seawater.

Membrane processing methods are a known desalination technology. Inmembrane processing methods, semi-permeable membranes are generallyemployed. Semi-permeable membranes are known as membranes that pass onlymolecules and ions of a specific size or smaller. For example, they aremembranes that pass the water but not the salt in seawater. When twosolutions of differing solute concentrations are brought into contactthrough a semi-permeable membrane, osmotic pressure is generated betweenthe two solutions. The solvent of the solution on the side of low soluteconcentration, that is, the side of low osmotic pressure, passes to theside of high solute concentration, that is, the side of high osmoticpressure. This phenomenon of osmosis theoretically continues until thestage where the osmotic pressure differential decreases to zero. Forexample, when seawater and water are brought into contact through asemi-permeable membrane, the water tends to pass through to the seawaterside and create a state of equilibrium.

The reverse osmosis process (RO process) and forward osmosis process (FOprocess) are known membrane processing methods utilizing suchsemi-permeable membranes.

The RO process is an osmotic technique whereby low-molecular-weightcomponents such as water are caused to move from the side of highosmotic pressure back through to the side of low osmotic pressure. Ahigh pressure exceeding the osmotic pressure differential of the twosolutions is applied to the side of high osmotic pressure in the ROprocess to bring about such reverse osmosis. For example, whenseparating water from seawater, seawater and water are brought intocontact through a semi-permeable membrane, and pressure exceeding theosmotic pressure differential between the seawater and the water,normally pressure greatly exceeding this osmotic pressure differential,is applied to the seawater side to cause the water in the seawater topass through to the water side.

By contrast, the FO process is a process whereby a solution with a highosmotic pressure draw (draw solution) is employed to artificiallygenerate an osmotic pressure differential between the two solutions andcause the water to migrate, as disclosed in Patent References 1 and 2.Specifically, a starting solution in the form of a feed solution (feedsolution) and a draw solution of higher osmotic pressure than the feedsolution are brought into contact through a semi-permeable membrane.When that is done, the osmotic pressure differential between the twosolutions causes the water in the feed solution to pass through to thedraw solution side. Subsequently, the solute component in the drawsolution is volatilized and recovered to separate out the water in thefeed solution. There are also cases where the concentrated feed solutionis separated out.

An example of a case of separating water from seawater will be describednext based on FIG. 1.

FIG. 1 shows an example of a seawater processing apparatus utilizing theFO process. The solid-line arrows show the flow of seawater or water 11separated from seawater, and the dotted lines show the flow of drawsolution or draw solution solute 12, respectively. Initially, seawater11 and draw solution 12 come into contact through a semi-permeablemembrane 13. The water in seawater 11 passes through semi-permeablemembrane 13 to the draw solution 12 side. Then, in stripping column 14,the solute component of the draw solution is volatilized from the drawsolution that has been diluted by the water of the seawater, therebyseparating out the water 16 and solute component 15 of the drawsolution. The solute component 15 of the draw solution is dissolved indraw solution that has been diluted in a gas absorber 17, and reused asdraw solution 12. Numeral 18 denotes a pressure gauge.

Gasifying and separating the solute component of the draw solution instripping column 14 requires high volatility. The solute component ofthe draw solution must also have a high degree of solubility so that itwill dissolve in the diluted draw solution. Further, the solutecomponent of the draw solution must naturally not pass through thesemi-permeable membrane. When these requirements are not met, there willbe problems in the FO apparatus or FO process in that the rate ofpermeation through the semi-permeable membrane by water from the feedsolution will be poor; the solute in the draw solution will end upleaking through the semi-permeable membrane and migrating to the feedsolution side, speeding up the rate; and the quantity of draw solutionsolute (stripping performance) remaining in the water obtained byvolatilizing the solute component from the draw solution that has beendiluted with water from the feed solution will be large.

Examples employing draw solutions in the form of solutions of ammoniaions and carbon dioxide, ammonia ion solutions, sulfur dioxidesolutions, and the like are known (Patent References 2 and 3). However,none fully combining the above high volatility, high solubility, andproperty of not passing through semi-permeable membranes has yet beenobtained.

PRIOR ART REFERENCES Patent References

-   [Patent Reference 1] U.S. Pat. No. 6,391,205-   [Patent Reference 2] U.S. Patent Application Publication    2005/0145568-   [Patent Reference 3] U.S. Pat. No. 3,171,799

SUMMARY OF INVENTION Problem to be Solved by the Invention

Patent Reference 2 achieves high volatility while ensuring a high degreeof solubility of solute components in the draw solution by using a drawsolution with ammonia and carbon dioxide as solutes, thereby achievingboth enhanced processing performance and enhanced stripping performancein the FO process. However, during examination, the present inventorsencountered a problem in that the ammonia ended up leaking out throughthe semi-permeable membrane in the diluting means. That is, in theprocess of Patent Reference 2, it was necessary to replenish largeamounts of ammonia in the FO apparatus. In Patent Reference 3, the rateof leakage through the semi-permeable membrane was reduced by employingsulfurous acid as a solvate component of the draw solution. However, itwas impossible to use anything but a solvate of low volatility toachieve high solubility in the draw solution, precluding adequatestripping performance.

The present invention has for its object to maintain a high passage ratethrough a semi-permeable membrane by water from a feed solution in an FOapparatus and in an FO process, control the migration of draw solutionsolute to the feed solution side due to leakage through thesemi-permeable membrane, and reduce the quantity of draw solution soluteremaining in the water obtained by volatizing the solute component fromthe draw solution that has been diluted with feed solution water(enhance the stripping performance).

Means of Solving the Problem

The present inventors conducted extensive research based on theseconditions, resulting the discovery that the above-stated problem couldbe solved by employing a draw solution containing an anion source and acation source such that the cation source had a molecular weight of 31or higher in an uncharged state and the cation source and the anionsource each had a Henry's law constant of 1.0×10⁴ (Pa/mol fraction) orhigher in a standard state.

Generally, the lower the Henry's law constant, the greater the tendencyto hydrate and the larger the hydration radius are thought to be(reference: Journal of Chemical & Engineering Data, 53 (2008),2873-2877, expanded equations (4), (5), and (6), InK_(aw)=ΔG_(hyd)/RT,where K_(aw)=dimensionless Henry's law constant, ΔG_(hyd)=hydration freeenergy (with the tendency to hydrate increasing with the magnitude ofthe negative value, R is Avogadro's constant, and T is absolutetemperature). Further, the greater the hydration radius, the slower theleakage of solute through the semi-permeable membrane is thought to be(reference: Desalination, 144 (2002), 387-392, FIG. 2(b), page 390, line7, left column to line 8, right column; reference: Journal of MembraneScience, 74 (1992), 05-103, FIGS. 4 and 5, page 98, line 34, left columnto page 101, line 9, left column). This naturally shows that the greaterthe level of hydration of a material, the less prone it is to volatilizein water (the lower the Henry's law constant), indicating that it isdifficult to achieve both a tendency not to leak through semi-permeablemembranes and a tendency to volatilize. Accordingly, it was thought thatit would be extremely difficult to solve the above-stated problem byadjusting the components of the draw solution. Surprisingly, however,extensive research by the present inventors resulted in the discoverythat the problem could be solved by employing draw solution solutecomponents in the form of a cation source and an anion source of greaterthan or equal to specified molecular weights and having specifiedHenry's law constants.

Specifically, the problem of the present invention was solved by thefollowing means.

<1> A forward osmosis apparatus, comprising a diluting means forbringing a feed solution and a draw solution comprising a cation sourceand an anion source in an ionized state into contact through asemi-permeable membrane and diluting the draw solution with waterseparated from the feed solution by means of the semi-permeablemembrane;

a separating means for separating the draw solution that has beendiluted by the diluting means into the cation source and anion sourceand into water; and

a dissolving means, returning the cation source and the anion sourcethat have been separated by the separating means to, and dissolving thecation source and anion source in, the draw solution that has beendiluted;

wherein the cation source in an uncharged state has a molecular weightof 31 or greater, and each of the anion source and cation source has aHenry's law constant of 1.0×10⁴ (Pa/mol·fraction) or greater in astandard state.

<2> The forward osmosis apparatus according to <1>, wherein the anionsource has an acid dissociation constant (pKa) of 6.0 to 7.0 as anuncharged material in a standard state.

<3> The forward osmosis apparatus according to <1> or <2>, wherein thecation source has a base dissolution constant (pKb) of 2.0 to 4.5 as anuncharged material in a standard state.

<4> The forward osmosis apparatus according to any one of <1> to <3>,wherein the cation source and/or anion source has a boiling point ofless than 100° C. at 1 atm as an uncharged material.

<5> The forward osmosis apparatus according to any one of <1> to <4>,wherein the cation source has a base dissolution constant (pKb) of 4.0to 4.5 as an uncharged material in a standard state.

<6> The forward osmosis apparatus according to any one of <1> to <5>,wherein the cation source has a Henry's law constant of 1.0×10⁵(Pa/mol·fraction) or greater in a standard state.

<7> The forward osmosis apparatus according to any one of <1> to <6>,wherein the cation source has a Henry's law constant of 1.0×10⁵(Pa/mol·fraction) or greater in a standard state.

<8> The forward osmosis apparatus according to any one of <1> to <7>,wherein each of the anion source and cation source in the draw solutionhas a concentration immediately prior to being contacted with thesemi-permeable membrane of 2.4 mol or more per kilogram of water.<9> The forward osmosis apparatus according to any one of <1> to <8>,wherein the anions are carbonic acid anions and/or hydrogen carbonateatoms.<10> The forward osmosis apparatus according to any one of <1> to <9>,wherein the molar ratio of the anion source and cation source is from1:1 to 1:2.<11> The forward osmosis apparatus according to any one of <1> to <10>,wherein the cation source is an amine compound.<12> The forward osmosis apparatus according to any one of <1> to <11>,wherein the cation source is one or more members selected from the groupconsisting of trimethyl amine, dimethyl ethyl amine, isopropyl amine,dimethyl amine, and diethyl amine.<13> The forward osmosis apparatus according to any one of <1> to <12>,wherein the cation source and anion source have a Henry's law constantof each of 1.00×10⁵ (Pa/mol·fraction) or greater, and the cation sourcehas a molecular weight of a range of from 45 to 74.<14> The forward osmosis apparatus according to any one of <1> to <13>,wherein the cation source is trimethyl amine or dimethyl ethyl amine,and the anion source is carbon dioxide.<15> The forward osmosis apparatus according to any one of <1> to <14>,wherein the diluted draw solution is separated into the cation sourceand anion source and into water by heating to a temperature notexceeding 90° C.<16> The forward osmosis apparatus according to any one of <1> to <15>,wherein the draw solution that is introduced by the diluting means has atemperature of ±5° C. of the feed solution introduced by the dilutingmeans.<17> The forward osmosis apparatus according to any one of <1> to <16>,which comprises a heat exchanger that warms the diluted draw solutionusing, as a heat source, at least one among the water, cation source,and anion source obtained by the separating means.<18> The forward osmosis apparatus according to any one of <1> to <17>,which comprises a heat exchanger that cools at least one among thecation source and anion source obtained by the apparatus of thedissolving means or the separating means using the feed solution priorto dilution as a cooling source.<19> The forward osmosis apparatus according to any one of <1> to <18>,wherein the difference between the maximum temperature of the separatingmeans and the minimum temperature of the dissolving means is less than35° C.<20> The forward osmosis apparatus according to any one of <1> to <19>,wherein the forward osmosis apparatus is a water purification apparatusthat separates the draw solution diluted by the diluting means into acation source and anion source and into water, and recovers the watercomponent as a target product.<21> The forward osmosis apparatus according to <20>, wherein the feedsolution is seawater.<22> The forward osmosis apparatus according to any one of <1> to <21>,wherein the forward osmosis apparatus is a concentrating device thatrecovers the feed solution that has been concentrated after bringing thefeed solution and draw solution into contact through the semi-permeablemembrane as a target product.<23> A draw solution for a forward osmosis process, comprising an anionsource and a cation source, wherein the cation source has a molecularweight of 31 or greater in an uncharged state, and each of the anionsource and cation source in water has a Henry's law constant of 1.0×10⁴(Pa/mol·fraction) or greater in a standard state.<24> The draw solution for a forward osmosis process according to <23>,wherein the acid dissolution constant (pKa) of the anion source as anuncharged material in a standard state is 6.0 to 7.0.<25> The draw solution for a forward osmosis apparatus according to <23>or <24>, wherein the anion source has an acid dissolution constant (pKa)of 6.0 to 7.0 as an uncharged material in a standard state.<26> The draw solution for a forward osmosis apparatus according to anyone of <23> to <25>, wherein the cation source in water has a Henry'slaw constant of 3.0×10⁴ (Pa/mol fraction) or greater in a standardstate.<27> The draw solution for a forward osmosis apparatus according to anyone of <23> to <26>, wherein each of the anion source and the cationsource has a concentration of 2.4 mol or more per kilogram of water.<28> The draw solution for a forward osmosis apparatus according to anyone of <23> to <27>, wherein the anions are carbonic acid ions and/orhydrogen carbonate ions, and the cation source is one or more membersselected from the group consisting of trimethyl amine, dimethyl ethylamine, isopropyl amine, dimethyl amine, and diethyl amine.<29> A forward osmosis process, comprising:

bringing a feed solution and a draw solution comprising a dissolvedcation source and an anion source into contact through a semi-permeablemembrane and diluting the draw solution with a liquid separated from thefeed solution by the semi-permeable membrane;

separating the diluted draw solution into the cation source and anionsource and into water; and

returning the separated cation source and anion source to, anddissolving the separated cation source and anion source in, the diluteddraw solution;

wherein the molecular weight of the cation source in an uncharged stateis 31 or greater and the Henry's law constant in water in a standardstate of each of the anion source and the cation source is 1.0×10⁴(Pa/mol·fraction) or greater.

Effect of the Invention

The FO apparatus and FO process of the present invention make itpossible to maintain a high permeation rate, inhibit leakage of drawsolution solute through the semi-permeable membrane and migration to thefeed solution side, and reduce the quantity of draw solution soluteremaining in water obtained by volatilizing solute components from thedraw solution that has been diluted with water from the feed solution(enhance stripping performance). As a result, it becomes possible toutilize the FO apparatus and implement the FO process withoutreplenishing draw solution solute components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example showing the configuration of a conventional FOapparatus.

FIG. 2 is an example showing the configuration of the FO apparatusemployed in an embodiment of the present application.

FIG. 3 is a graph of the correlation between the molecular weight of thecation source and leakage.

FIG. 4 is a graph showing the correlation between the Henry's lawconstant multiplied by the molecular weight and leakage.

FIG. 5 is a simulation flow diagram employed in an embodiment of thepresent application.

MODES OF CARRYING OUT THE INVENTION

The contents of the present invention are described in detail below. Inthe present description, numeric values preceding and succeeding theword “to” are employed to mean the maximum and minimum values,inclusive, of a range.

In the present description, the term “anion source” refers to asubstance that generates anions when dissolved in water, and the term“cation source” refers to a substance that produces cations whendissolved in water. Accordingly, the anion source and cation source arenormally uncharged materials. In the present invention, the term“standard state” means 25° C. and 10⁵ Pa (about 1 atmosphere).

The FO apparatus of the present invention comprises a diluting means forbringing a feed solution and a draw solution comprising a cation sourceand an anion source in an ionized state into contact through asemi-permeable membrane and diluting the draw solution with waterseparated from the feed solution by means of the semi-permeablemembrane; a separating means for separating the draw solution that hasbeen diluted by the diluting means into the cation source and anionsource and into water; and a dissolving means, returning the cationsource and the anion source that have been separated by the separatingmeans to, and dissolving the cation source and anion source in, the drawsolution that has been diluted; wherein the molecular weight of thecation source in an uncharged state is 31 or greater and the Henry's lawconstant of each of the anion source and cation source is 1.0×10⁴(Pa/mol·fraction) or greater in a standard state. The present inventionwill be described in detail below.

The FO apparatus of the present invention refers to an apparatus thatconducts separation, concentration, filtration, and the like by an FOprocess. That is, it is not specifically prescribed so long as itcomprises a method of artificially generating an osmotic pressuredifferential between two solutions with a draw solution of high osmoticpressure to cause water to migrate. For example, the case where anosmotic pressure differential is applied and pressure is applied toforce feed a liquid is also covered by the FO process of the presentinvention. Accordingly, in the FO apparatus of the present invention,the target product that is ultimately recovered can be the water that isseparated on the draw solution side or the liquid that is concentratedon the feed solution side.

When recovering the water that is separated on the draw solution side asthe target product, the FO apparatus of the present invention isdesirably a water purification device. The feed solution that isemployed is not specifically limited and can be suitably selected basedon the objective. Examples are seawater, brackish water, river water,water obtained from locations in the natural world such as lakes,swamps, and ponds, industrial waste water discharged by factories andvarious industrial facilities, common waste water discharged byhouseholds and common facilities, and microorganism culture solutions atwater treatment facilities. Of these, seawater is preferred due to itsready availability in stable, large quantities and the need forpurification.

When recovering the concentrated feed solution as the target product,the FO apparatus of the present invention is desirably a concentratingapparatus. In this case, examples of such feed solutions areconcentrated fruit juices and vegetable juices.

The FO apparatus of the present invention normally comprises a dilutingmeans, separating means, and dissolving means, as well as other means asneeded. The FO process of the present invention comprises diluting,separating, and dissolving, and is carried out by the FO apparatus ofthe present invention.

<The Diluting Means>

The diluting means is a process where the feed solution and the drawsolution are brought into contact through a semi-permeable membrane andthe semi-permeable membrane dilutes the draw solution with water that isseparated from the feed solution. This process can be conducted at 15 to40° C., for example. When the draw solution that is introduced by thediluting means is warmed by the feed solution, the solute in the drawsolution gasifies and potentially forms bubbles. Conversely, whencooled, the solute in the draw solution potentially precipitates. Thus,the temperature of the draw solution that is introduced is desirably ±5°C. that of the feed solution that is introduced by the diluting means.Further, the liquid feed pressure of the feed solution can be such thatdriving is conducted at a low pressure different from that in RO, suchas 1×10⁴ Pa to 5×10⁵ Pa.

The semi-permeable membrane is not specifically limited in terms of itsmaterial, shape, size, configuration, or the like, and can be suitablyselected based on the objective. For example, it can be a flat membrane,a spiral module employing a flat membrane, a hollow fiber module, or atubular module. The feed direction of the draw solution and feedsolution with respect to the surface of the membrane is not limited;feeding can be conducted antiparallel, parallel, or in a non-paralleldirection such as at an angle of 90°. The thickness of the feed flowroute as measured from the surface of the semi-permeable membrane is notspecifically limited, and can be from 10 μm to 10 mm, by way of example.The shallower the flow route the better because the permeationefficiency of water from the feed solution through the semi-permeablemembrane increases. However, clogging and the like also tend to occur,so a suitable thickness is selected.

Nor is the material of the semi-permeable membrane specifically limitedother than that it be capable of separating water and solute. Examplesare cellulose acetate, aromatic polyamides, aromatic polysulfones, andpolybenzoimidazoles. A polyamide or cellulose acetate with a degree ofacetyl group substitution of 2.50 to 2.95 is desirably selected.

The method of manufacturing the semi-permeable membrane is notspecifically limited other than that separation of water and solute bepossible. Examples are the non-solvent induced phase separation method(NIPS), thermally induced phase separation method (TIPS), interfacialpolymerization method, and solvent evaporation method. For example,various membrane-manufacturing methods can be conducted by the methodsdescribed in experimental membrane science methods in A Compilation ofArtificial Membranes (compiled by the Membrane Society of Japan), 1.2Methods for manufacturing macromolecular membranes. When manufacturing asemi-permeable membrane by the solvent evaporation method, a thinmembrane with a thickness of less than 1 μm is desirable. In that case,there is a risk of damaging the semi-permeable membrane in the course ofpeeling the thin membrane off the coating substrate (including filmforms). Thus, the method of coating a sacrificial layer on the coatingsubstrate in advance and peeling off one membrane for each sacrificiallayer can be adopted. Here, the term “sacrificial layer” refers to alayer that spontaneously peels off or dissolves when immersed insolvent, heated, or the like.

A membrane that is employed as a RO membrane can be employed as is, orin an improved form, as the semi-permeable membrane in addition to afilm manufactured as a FO membrane. However, a FO membrane in whichdeterioration in permeation performance has been inhibited by internalconcentration polarization is desirably selected. As specific examples,the Expedition built-in membrane and the X-Pack built-in membrane, bothmade by Hydration Technology Innovation, can be employed.

The Feed Solution

The feed solution is not specifically limited. It can be suitablyselected based on the objective. Examples are seawater, brackish water,river water, water obtained from locations in the natural world such aslakes, swamps, and ponds, industrial waste water discharged by factoriesand various industrial facilities, common waste water discharged byhouseholds and common facilities, microorganism culture solutions atwater treatment facilities, membrane bioreactor (MBR) microorganismculture solutions, fruit juices, and vegetable juices. Of these,seawater is preferred due to its ready availability in stable, largequantities and the need for purification.

The feed solution can be subjected to various pretreatments prior toprocessing by the diluting means. Examples are common water treatmentmethods such as flocculent treatment, sedimentation treatment, andfiltration treatments such as sand filtration and microfiltration. Afurther example is the method of dilution by prior use of the feedsolution as a draw solution in another forward osmosis apparatus, suchas the osmotic dilution described in the Journal of Membrane Science,362 (2010), pp. 417-426. Reducing the solute concentration of the feedsolution by that method can be anticipated to have effects such asenhancing the dilution rate in the forward osmosis apparatus of thepresent invention and greatly reducing the energy input.

The Draw Solution

The draw solution comprises solutes in the form of an anion source and acation source each having a Henry's law constant of 1.0×10⁴(Pa/mol·fraction) or greater. The Henry's law constant is a physicalproperty indicating the relation between the mol fraction of a substanceand the saturation vapor partial pressure in a solution in which a largequantity of a solution has been dissolved in water. The greater thisconstant, the greater the volatility in the aqueous solution and thelower the solubility indicated. It is described in books: ChemistryHandbook (published by Maruzen K.K.) and Gas Absorption, Supplemented(published by Kagaku Kogyo K.K.); in the literature: Compilation ofHenry's Law Constants for Inorganic and Organic Species of PotentialImportance in Environmental Chemistry (hyperlink:http://www.mpch-mainz.mpg.de/˜sander/res/henry.html); and the like. Whenthe solute becomes charged (ions), since only the uncharged component ofthe solute correlates to the saturation vapor pressure, the Henry's lawconstant is a physical property that indicates the relation between themol fraction of the uncharged component of the solute and the saturationvapor partial pressure. Thus, when the anion source and cation sourcebecome charged in the form of anions and cations based on the pH of theaqueous solution, the saturation vapor pressure drops, that is, thesolubility increases. Since an anion source and a cation source withhigh Henry's law constants of 1.0×10⁴ (Pa/mol·fraction) or greater areemployed in the present invention, solubility is low in the unchargedstate. However, when the anion source and the cation source are broughttogether to adjust the pH, the ratio of each of the charged materialsincreases and their solubility increases markedly relative to the caseof the solution alone.

The draw solution comprises an anion source and a cation source. Here,the anion source and the cation source are contained in the drawsolution in an ionized state.

The content of the anion source and cation source in the draw solutionis not specifically limited and can be suitably selected based on theobjective.

In the diluting means, high concentrations are desirable from theperspective of increasing the rate of separation of water from the feedsolution. For example, the total concentration of dissolved solute inthe draw solution just prior to the diluting means is desirably 4.8 molor more per kilogram of water. There is a risk of inviting deteriorationof the semi-permeable membrane and the like if the pH of the solution isextremely high or low, so the pH is desirably 4 to 10. When employingcarbon dioxide as the anion source, a mixing ratio that yields pH 8 orhigher is desirable to increase solubility. The molar ratio of the anionsource and cation source is desirably 1:1 to 1:2, preferably 1:1.2 to1:1.7. A suitable value is selected for this ratio based on the speciesof anion and cation.

In the separating means, it is desirable for the diluted draw solutionto be of low concentration from the perspectives of increasingseparation efficiency and reducing the energy required for separation.Thus, the dilution rate in the diluting means is desirably high.However, when the osmotic pressure differential between the diluted drawsolution and the feed solution decreases, the permeation rate of waterthrough the semi-permeable membrane in the diluting means drops sharply,so a suitable concentration and dilution rate are selected.

The Cation Source

The molecular weight of the cation source in the present invention is 31or greater in an uncharged state and the Henry's law constant in astandard state is 1.0×10⁴ (Pa/mol·fraction) or greater. Employing such acation source with the anion source makes it possible to maintain a highpermeation rate of water through the semi-permeable membrane and toinhibit leakage of the solute in the draw solution through thesemi-permeable membrane and migration to the feed solution side in an FOapparatus. It also reduces the amount of draw solution solute remainingin the water obtained by volatilizing the solute component from the drawsolution that has been diluted with water from the feed solution(enhances stripping performance).

The molecular weight of the cation source in an uncharged state isdesirably 45 to 74, preferably 45 to 62.

The Henry's law constant of the cation source in a standard state isdesirably 1.0×10⁵ (Pa/mol·fraction) or greater. The upper limit is notspecifically set. By way of example, it can be 1.0×10⁷ (Pa/mol·fraction)or lower. Setting it to within such a range makes it possible to raisethe volatility in the separating means, and as a result, reduce thequantity of water that ends up unnecessarily volatilizing in theseparating means. As a result, it becomes possible to keep down theenergy required for separation.

When the base dissociation constant (pKb) of the cation source as anuncharged material is excessively low in the standard state, there areproblems in that the volatilization decreases in the separating means,the quantity of cation source remaining in the separated waterincreases, and the energy required for separation increases. Conversely,when the pKb is excessively high, there are problems in that the pH inthe absorber drops, the quantity of anion source dissolving decreases,and the permeation rate of water through the semi-permeable membranedecreases in the diluting means. Thus, the pKb must be suitably selectedand is desirably 2.0 to 4.5, preferably 4.0 to 4.5.

Keeping the molecular weight, Henry's law constant, and pKb of thecation source within the above suitable ranges more effectively inhibitsthe cation source from leaking through the semi-permeable membrane andmigrating to the feed solution side, raises the separation efficiency ofthe separating means, keeps down the energy required for separation, andraises the efficiency of the diluting means.

The boiling point of the cation source as an uncharged material at 1 atmis desirably less than 100° C., preferably not greater than 50° C.Remaining within this range inhibits the condensation during operationof gas in the cation source that has been separated in the separatingmeans. The lower limit is not specifically set. For example, it can be−20° C. or higher.

The concentration of the cation source in the draw solution just beforebeing brought into contact with the semi-permeable membrane is desirably2.4 mol or more per kilogram of water, preferably 3.0 mol/kg or more.Adopting such means makes it possible to raise the permeation rate ofwater through the semi-permeable membrane in the diluting means. Theupper limit is not specifically limited, and can normally be 8.0 mol/kgor less.

The cation source need not be specifically limited to remain within thespirit and scope of the present invention. However, it is desirably anamine compound and can be denoted by general formula (I) below.R_(n)—NH_((3-n))  General formula (I):(In general formula (I), R denotes a linear or branched aliphatic groupwith 1 to 4 carbon atoms and n denotes 1, 2, or 3.)

In general formula (I), R desirably denotes a methyl group with 1 carbonatom and n desirably denotes 3.

Specific examples are one or more selected from among trimethyl amine,dimethyl ethyl amine, isopropyl amine, dimethyl amine, diethyl amine,n-propyl amine, ethyl amine, n-butyl amine, isopropyl amine, isobutylamine, t-butyl amine, pyrrolidine, methyl amine, ethyl methyl amine,methyl n-propyl amine, methyl isopropyl amine, pentyl amine, dimethyln-propyl amine, dimethyl isopropyl amine, ethyl n-propyl amine, ethylisopropyl amine, and diethyl methyl amine; more preferably one or moreselected from among trimethyl amine, dimethyl ethyl amine, isopropylamine, dimethyl amine, and diethyl amine; and still more preferably,trimethyl amine or dimethyl ethyl amine.

The Anion Source

The anion source in the present invention has a Henry's law constant ina standard state of 1.0×10¹ (Pa/mol·fraction) or greater. Employing suchan anion source in combination with the cation source maintains a highpermeation rate and inhibits the solute in the draw solution fromleaking through the semi-permeable membrane and migrating to the feedsolution side in a FO apparatus, as well as reducing the amount of drawsolution solute remaining in the water obtained by volatilizing thesolute component from the draw solution that has been diluted with feedsolution water (enhancing stripping performance).

The base dissociation constant (pKb) of the anion source as an unchargedmaterial in a standard state is desirably 2.0 to 4.5, preferably 4.0 to4.5. Keeping it within such a range has the effects of causingpreferential volatilization and separation over cations in theseparating means, enhancing overall separation efficiency, andinhibiting leakage of the anion source through the semi-permeablemembrane and migration to the feed solution side in the diluting means.

The boiling point of the anion source as an uncharged material at 1 atmis desirably less than 100° C., preferably not greater than 50° C., andmore preferably, not greater than 0° C. Keeping it within such a rangehas the effects of causing preferential volatilization and separationover cations in the separating means and enhancing overall separationefficiency. The lower limit is not specifically set. However, it isnormally −100° C. or higher.

The Henry's law constant of the anion source in a standard state ispreferably 1.0×10⁷ (Pa/mol·fraction) or greater. Keeping it within sucha range has the effects of causing preferential volatilization andseparation over cations in the separating means and enhancing overallseparation efficiency. The upper limit is not specifically set, but isnormally 1.0×10¹⁰ (Pa/mol·fraction) or lower.

In the standard state, the acid dissociation constant (pKa) of the anionsource as an uncharged material is desirably 6.0 to 7.0.

The concentration of the anion source in the draw solution Dust beforebeing brought into contact with the semi-permeable membrane is desirably2.4 mol or more per kilogram of water, preferably, 2.8 mol/kg or more.Employing such a means increases the permeation rate of water throughthe semi-permeable membrane in the diluting means. The upper limit isnot specifically set, but is normally 8.0 mol/kg or less.

The anion source need not be specifically limited to remain within thespirit and scope of the present invention. Examples are carbon dioxide(CO₂), carbonic acid (H₂CO₂), and sulfur dioxide (SO₂). Of these, carbondioxide (CO₂) is particularly desirable from the perspectives of highvolatility, stability, low reactivity, and ready availability.Accordingly, a mixture of carbonic acid ions and hydrogen carbonate ionsis desirable as the anions. In some cases, complex ions such ascarbamate ions can be incorporated.

<The Dissolving Means>

The dissolving means returns and dissolves the volatile solutes of thedraw solution that have been separated by the separating means to thedraw solution that has been diluted by the diluting means. Thedissolving means is not specifically limited and can be suitablyselected based on the objective from among those apparatus that arecommonly employed to absorb gases. For example, the apparatus, parts,conditions, and the like that are described in the book: Gas Absorption,Supplemented (published by Kagaku Kogyo K.K.), pp. 49 to 54, pp. 83 to144, can be optionally employed. Specific examples are methods employingabsorbers, packed columns, tray columns, spray columns, and fluid packedcolumns; liquid film crossflow contact methods; high-speed rotating flowmethods; and methods utilizing mechanical forces. It is also possible toconstruct and cause the absorption of thin gas and liquid layers usingmicrofluidic control devices such as microreactors and membranereactors. Structures similar to the absorbers employed in heat pumpssuch as ammonia absorption chillers can also be employed.

The packings that are packed into packed columns can be structuredpackings or unstructured packings. For example, the packings describedin the book: Gas Absorption, Supplemented (published by Kagaku KogyoK.K.), pp. 221 to 242 can be optionally selected.

Structural parts and materials such as packings, columns, distributors,and supports are not specifically limited and can be suitably selectedbased on the objective. Examples are steel-based materials such asstainless steel and aluminum killed steel; nonferrous materials such astitanium and aluminum; ceramics such as glass and alumina; and materialssuch as carbon, synthetic polymers, and rubbers. To efficiently absorbthe anion source and cation source gases in the dissolving means, thepresence of a cooling function is desirable. The cooling function can ofan integrated form with heat exchanging elements that run throughout thedissolving means, or in the form of one or multiple heat exchangersattached to one part of the dissolving means. From the perspective ofefficient dissolution, an integrated form is desirably selected.Multiple types of gas absorbers can be employed in the dissolving means.

A low operating temperature in the dissolving means generally enhancesdissolving efficiency. However, since an immense amount of coolingenergy is required to get below ordinary temperature, from theperspective of the amount of energy required, a temperature of ordinarytemperature or above, or from ordinary temperature to 15° C. aboveordinary temperature is desirable. Here, the term “ordinary temperature”means ambient temperature.

<The Separating Means>

In the separating means, the volatile solute in the draw solution isseparated from the draw solution that has been diluted by the dilutingmeans and a target solution such as water or a concentrated feedsolution is obtained.

The separating means need not be specifically limited to remain withinthe spirit and scope of the present invention. It can be suitablyselected based on the objective. Examples are the stripping columns,stripping apparatus, membrane processing units, microreactors, and othermicrofluidic control devices commonly employed in stripping. Of these,stripping columns are preferred.

When employing a stripping column, the heating temperature is desirablynot greater than 90° C., preferably not greater than 75° C. Employingsuch a means permits the use of a heat medium of low utility value asthe energy source, thereby greatly enhancing economy. The lower limit isnot specifically established, but is normally 30° C. or higher.

The method of heating in the separating means can be suitably selectedso long as it permits heating of the diluted draw solution. From theperspective of economy, it is desirable to select a method of heatingthat utilizes waste heat of less than 100° C., not electrical heating orhigh-temperature heat sources of 100° C. or higher. Specifically, powerplants such as thermal power plants and nuclear power plants,incineration furnaces, steel-making industry and petrochemical industryplants, sunlight collection systems employing mirrors and lenses, andthe like can be selected. The selection of a power plant is desirablefrom the perspective of the quantity of waste heat.

The stripping column is not specifically limited and can be suitablyselected based on the objective. Examples are tray columns and packedcolumns.

Examples of tray columns are structures described in the booksDistillation Technology (published by Kagaku Kogyo K.K.), pp. 139 to 143and Chemical Engineering Explained (published by Baifukan), pp. 141 to142. Specific examples are bubble cap trays, valve trays, and poroussheet (sieve) trays.

The packings that are packed into packed columns can be structuredpackings or unstructured packings. For example, the packings describedin Chemical Engineering Explained (published by Baifukan), pp. 155 to157, and Gas Absorption, Supplemented (published by Kagaku Kogyo K.K.),pp. 221 to 242 can be optionally selected.

For example, the membrane distillation unit described in a scientificpaper in the Journal of Membrane Science, Vol. 124, Issue 1, pp. 1 to 25and the like can be employed as a membrane processing unit.

For example, the reactor described in the book Techniques andApplications of Microchemical Chips (published by Maruzen) can beemployed as a microreactor.

When employing carbon dioxide as the anion source, slower reactions thanin hydrocarbon-based stripping columns, such as hydration anddehydration, are involved and an extremely long time is required toachieve gas-liquid equilibrium. Thus, to get the separating means tofunction efficiently, it is desirable to keep the solution in thevarious packings and trays for longer than the recommended retentiontime calculated for a hydrocarbon system. Examples of common methodsthat can be adopted to lengthen the retention time are increasing thediameter of the stripping column, selecting a tray column, and adjustingthe pore size.

<Other Processes>

Examples of other processes are control processes and driving processes.These are carried out by control means and driving means.

The control means is not specifically limited other than it be capableof controlling the operation of each means, and can be suitably selectedbased on the objective. Examples are devices such as sequencers andcomputers.

<Required Energy>

In the forward osmosis process of the present invention, relativelyspecific conditions such as the anion source and cation source can beselected to greatly reduce the energy required. In particular, since alow quality heat source can be employed as the energy source utilized inthe forward osmosis process, great significance is achieved inmanufacturing. Here, the term “low quality energy” refers to energy thatcannot be used in common applications, or that affords poor efficiency.For example, for heat of less than 100° C., the electricity conversionefficiency of turbines and the like is poor, and the utility value ofsuch heat is low. The forward osmosis process of the present inventionmainly requires feed energy and heating energy in the separating means.However, since the feed pressure is low, in contrast to RO, heatingenergy accounts for most of the energy in the separating means. However,when the temperature in the dissolving means is lower than ordinarytemperature, the cooling temperature becomes immense. Thus, it must beset to greater than or equal to ordinary temperature. Thus, to reducethe overall energy, optimal conditions must be selected for the entiresystem, not just the separating means.

When a thermodynamic simulation of the forward osmosis system of thepresent invention was simulated, reducing the quantity of water thatended up being needlessly volatilized with the anion source and cationsource in the separation element was found to effectively keep down theenergy in the separating means. In particular, when carbon dioxide wasemployed as the anion source, separation occurred with the anion sourcevolatilizing preferentially in the initial period in the separatingmeans. Thus, the ease with which the remaining cation source volatilizedand separated was important. That is, the selection criterion for thecation source is not its volatilization energy; it is important toselect cations with a high Henry's law constant and a high pKb. In thedissolving means, it is necessary to efficiently dissolve the carbondioxide, but as set forth above, an immense amount of cooling energybecomes necessary when the temperature in the dissolving means is madeexcessively low. Thus, a range of from ordinary temperature to ordinarytemperature plus 15° C. is desirable. To enhance the dissolutionefficiency of carbon dioxide, a high pH is desirable in the drawsolution. Thus, the pkb of the cation source is desirably low. The factthat the pKb that is required in the cation source runs counter to whatis needed in the separating means and dissolving means in this mannerindicates that a suitable range will be present. Specifically, from theperspective of the energy required, a pKb of 3.2 to 4.5 and a Henry'slaw constant of 1.0×10⁵ (Pa/mol·fraction) or greater are desirable, anda pKb of 4.0 to 4.5 and a Henry's law constant of 3.0×10⁵(Pa/mol·fraction) or greater are preferred in the cation source. Morespecifically, trimethyl amine or dimethyl ethyl amine is desirablyselected, for example.

In addition to the above, the techniques described in U.S. Pat. No.6,391,205, US Patent Application Publication 2005/0145568, andInternational Publication No. WO2007/146094 can be adopted in thepresent invention to an extent that does not depart from the spirit orscope of the present invention.

Embodiments

The present invention is described more specifically below throughembodiments. The materials, quantities employed, ratios, processingcontents, processing procedures and the like that are indicated in theembodiments below can be suitably modified within departing from thespirit or scope of the present invention. Accordingly, the scope of thepresent invention is not limited by the specific examples given below.

In the present embodiments, the following substances were employed asthe cation source and anion source.

The Henry's law constant is a value based on the Compilation of Henry'sLaw Constants for Inorganic and Organic Species of Potential Importancein Environmental Chemistry (hyperlink:http://www.mpch-mainz.mpg.de/˜sander/res/henry.html). For substances forwhich multiple values were listed, extreme values were eliminated andthe average value was employed. It is denoted in units ofPa/mol·fraction. The molecular weight is the molecular weight of therespective compound in an uncharged state. The pKa and pKb are the usualvalues based on the literature, such as the Chemical Handbook, BasicEdition, ed. by the Chemical Society of Japan, Maruzen. The boilingpoint is a value based on 1 atm.

TABLE 1 pKa Boiling Henry's Molecular or point Solute (Uncharged state)constant weight pKb [° C.] Cation Trimethylamine 6.0E+05 60.11 4.21 3Dimethyl ethyl amine 4.0E+05 74.14 4.17 36 Isopropyl amine 2.6E+05 60.113.37 33 Diethyl amine 2.2E+05 74.14 2.98 55.5 Dimethyl amine 1.3E+0546.08 3.27 7 Ethyl amine 8.0E+04 46.08 3.3 17 Propyl amine 7.8E+04 60.113.33 48 Pentyl amine 1.4E+05 88.16 3.4 104 Ammonia(comparing) 9.8E+0418.03 4.75 −33 Ethylene- 9.7E+00 61.1 3.92 116 diamine(comparing) AnionCO₂ 1.6E+08 44.01 6.35 −79 SO₂ 4.6E+06 64.07 1.81 −10 Trifluoroacetic7.7E+02 114.03 0.3 72 acid(comparing)Rate of Leakage of Solute Through Semi-Permeable Membrane

The rate at which just the cation source permeated through thesemi-permeable membrane from an aqueous solution was analyzed in a modelexperiment of the rate of leakage of the solute in the diluting means.Of the anion source and its charged materials and the cation source andits charged materials, the uncharged cation source was known to pass themost readily through a semi-permeable membrane when carbon dioxide wasemployed as the anion source in the present invention. Thus, theanalysis was effective in that the cation source was selected.

Concentrations of 0.4 mol of ammonia (made by Kanto Chemical Co., Inc.),dimethyl amine (made by Tokyo Chemical Industry Co., Ltd.), ethyl amine(made by Tokyo Chemical Industry Co., Ltd.), propyl amine (made by WakoPure Chemical Industries, Ltd.), trimethyl amine (made by Wako PureChemical Industries, Ltd.), and diethyl amine (made by Wako PureChemical Industries, Ltd.) per kilogram of pure water were prepared asdraw solutions. Pure water was employed as the feed solution. AnExpedition built-in membrane made by Hydration Technology Innovations(referred to as an “HT membrane” hereinafter) was employed as thesemi-permeable membrane. A 100 mL quantity of feed solution and 100 mLof draw solution were pumped at a flow rate of 20 mm/second with aPeri-Star pump and the feed solution and draw solution were brought intocontact through the semi-permeable membrane in a cell holding asemi-permeable membrane (HT membrane). At the time, the membrane contactsurface area was 280 mm² and the two flows were parallel and identicallyoriented. The two solutions were contacted while being fed for 30minutes, after which the concentration of each cation source in the feedsolution was measured and the semi-permeable membrane permeation ratewas estimated. The pH and electrical conductivity of the feed solutionswere measured and the cation concentration of each cation sourcecontained was estimated from a calibration curve. The results are givenin Table 2. FIG. 3 shows the correlation with the molecular weight ofthe cation source and FIG. 4 shows the correlation with the value of themolecular weight multiplied by the Henry's law constant of the cationsource.

TABLE 2 Rate of leakage Molecular Standard Henry's constant weight Henry× MW Average deviation Cation source Abbreviation [Pa/molfrac] [g/mol][gPa/mol molfrac] [umol/mm²hr] — Ammonia NH₃ 9.8E+04 17 1.66E+06 6.920.51 Dimethyl amine NMe₂ 1.3E+05 45.08 5.86E+06 2.85 0.26 Ethyl amineNEt 8.0E+04 45.08 3.59E+06 3.30 0.48 Propyl amine NPr 7.8E+04 59.114.61E+06 3.21 0.31 Trimethylamine NMe₃ 6.0E+05 59.11 3.53E+07 1.36 0.2Diethyl amine NEt₂ 2.2E+05 73.14 1.58E+07 1.96 0.34

As shown in Table 2 and FIG. 3, the results indicated that the use of acation source with a molecular weight of 31 or greater suppressed thesemi-permeable membrane permeation rate. As shown in FIG. 4, theselection of a cation source with a high Henry's law constant inaddition to a high molecular weight better suppressed the semi-permeablemembrane permeation rate.

Adjustment of Draw Solution

Draw solutions with the compositions shown in Table 3 were prepared. ForCO₂, gas absorption was conducted with carbon dioxide (using a CO₂ gascylinder) while cooling a cation source aqueous solution prepared inadvance. Trifluoroacetic acid (made by Wako Pure Chemical Industries,Ltd.) was added in liquid form. For SO₂, a sulfurous acid aqueoussolution (made by Wako Pure Chemical Industries, Ltd.) was employed inthe preparation.

Adjustment of Feed Solution

A 0.1 weight % solution (0.1% BSA) of bovine serum albumin (made by WakoPure Chemical Industries, Ltd.) or a 0.6 M solution (0.6M NaCl) ofsodium chloride (made by Wako Pure Chemical Industries, Ltd.) wasemployed as the feed solution.

In the FO apparatus shown in FIG. 2, the diluting means and separatingmeans were independently assembled. In FIG. 2, the solid line arrowsindicate the flow of the feed solution or of water separated from thefeed solution and the dotted lines indicate the flow of the drawsolution or of the draw solution solute, respectively. The present FOapparatus is comprised of a diluting means 21, a dissolving means 22,and a separating means 23.

Initially, as a diluting means, 200 mL of feed solution and 200 mL ofdraw solution were brought into contact through a semi-permeablemembrane 24 (HT membrane) in diluting means 21 (membrane contact surfacearea 280 mm²). Each solution was pumped at a flow rate of 20 mm/secondin parallel with the same orientation using a Peri-Star pump. The rateat which the water in the feed solution permeated to the dissolvingmeans 22 side through semi-permeable membrane 24 was analyzed bymeasuring the weights of the feed solution and draw solution in realtime. The cation concentration that had leaked out from the drawsolution and was contained in the feed solution after flowing for threehours was quantified with various amine electrodes or by gaschromatography, and the leakage rate was computed. The amine electrodesemployed in amine quantification were obtained by replacing the internalliquid of commercial ammonia electrodes (made by DKK-TOA Corporation,comprising ammonia composite electrodes AE-2041 connected to a portableion meter IM-32P) with a chloride aqueous solution of the amine to bemeasured (for example, 50 mM trimethyl amine chloride aqueous solution).The measurement method consisted of adjusting the pH of the sampleliquid with NaOH aqueous solution and then measuring the electromotiveforce in a stable state. Calibration curves of amine concentration andelectromotive force were prepared for various aqueous solutionconcentrations of various measurement controls and the amineconcentrations of the samples were then estimated.

Next, a stripping column was employed as a separating means to measurethe amount of remaining solute. A dual-tube configuration strippingcolumn with a built-in structured packing (Laboratory Packing EX, madeby Sulzer Chemtech: referred to as “Labpack” hereinafter) was employedas the stripping column. While heating the bottom with a mantle heater,the draw solution that had been diluted by water from the feed solutionwas continuously fed from the top of the stripping column. In thestripping column of separating means 23, the solute component of thedraw solution was volatilized to separate the water from the solutecomponent of the draw solution. At the time, the top of the column wasconnected through a cooling element to a vacuum pump and the internalpressure was automatically regulated to 1.0×10⁴ Pa. During three hoursof operation, a suitable quantity of separated liquid was sampled fromthe bottom of the column and the concentration of cations remaining inthe solution discharged from the bottom of the stripping column while ina steady state was measured. The measurement was conducted by the samemethod as in the above diluting means.

Permeation Flow Rate

The semi-permeable membrane permeation flow rate of water from the feedsolution was measured in the above process.

◯: 300 μmol/mm² hr or more

Δ: 30 μmol/mm² or more but less than 300 μmol/mm²

X: less than 30 μmol/mm²

Above, Δ or better indicates a practical level.

The Leakage Rate

The leakage of solute component through the semi-permeable membrane tothe draw solution was measured in the above process.

The following evaluation was conducted.

◯: less than 1 μmol/mm²

Δ: 1 μmol/mm² or more but less than 5 μmol/mm²

X: more than 5 μmol/mm²

Above, Δ or better indicates a practical level.

Amount of Remaining Solute

The amount of draw solution remaining in the water after separation ofthe cation source and anion source from the draw solution was measured.The following evaluation was conducted.

⊚: less than 50 μM

◯: 50μ or more but less than 200 μM

Δ: 200 μM or more but less than 1 mM

X: 1 mM or more

Above, Δ or better indicates a practical level.

TABLE 3 Species Amount of Species Content[mol/ (Uncharged Content[mol/Leakage remaining (Uncharged state) kg H₂O] state) kg H2O] Feed solutionFlow rate rate solute Example 1 Trimethylamine 4.2 CO₂ 3 0.1% BSA ◯ ◯ ⊚Example 2 Trimethylamine 4.2 CO₂ 3 0.6M NaCl ◯ ◯ ⊚ Example 3 Dimethylethyl amine 4.2 CO₂ 3 0.1% BSA ◯ ◯ ⊚ Example 4 Dimethyl ethyl amine 4.2CO₂ 3 0.6M NaCl ◯ ◯ ⊚ Example 5 Isopropyl amine 4.2 CO₂ 3 0.1% BSA ◯ ◯ ◯Example 6 Isopropyl amine 4.2 CO₂ 3 0.6M NaCl ◯ ◯ ◯ Example 7 Diethylamine 4.2 CO₂ 3 0.6M NaCl ◯ ◯ ◯ Example 8 Propyl amine 4.2 CO₂ 3 0.6MNaCl ◯ Δ ◯ Example 9 Ethyl amine 4.2 CO₂ 3 0.6M NaCl ◯ Δ ◯ Example 10Pentyl amine 4.2 CO₂ 3 0.6M NaCl ◯ ◯ Δ Example 11 Trimethylamine 1.4 CO₂1 0.1% BSA Δ ◯ ⊚ Example 12 Trimethylamine 1.4 CO₂ 1 0.6M NaCl Δ ◯ ⊚Example 13 Dimethyl ethyl amine 1.4 CO₂ 1 0.1% BSA Δ ◯ ⊚ Example 14Trimethylamine 4.2 CO₂ 2 0.6M NaCl ◯ Δ ⊚ Comparative Trimethylamine 4.2— — 0.6M NaCl X X ⊚ Example 1 Comparative — — SO₂ 0.5 0.1% BSA Δ ◯ XExample 2 Comparative Ammonia 4.2 CO₂ 3 0.6M NaCl ◯ X ◯ Example 3Comparative Ammonia 1.4 CO₂ 1 0.6M NaCl Δ X ◯ Example 4 ComparativeEthylenediamine 4.2 CO₂ 3 0.1% BSA ◯ Δ X Example 5 ComparativeTrimethylamine 0.75 Trifluoro 0.45 0.1% BSA ◯ Δ X Example 6 acetic acid

As will be clear from the above table, when the cation source employedhad a Henry constant of less than 1.0×10⁴ (Pa/mol·fraction) or themolecular weight of the cation source was less than 31 (ComparativeExamples 3 to 5), the leakage rate and quantity of remaining solutedeteriorated. When the Henry's law constant of the anion source was lessthan 1.0×10⁴ (Pa/mol·fraction) (Comparative Example 6), the quantity ofremaining solute deteriorated.

When the molecular weight of the cation source in a uncharged state was31 or greater and the Henry's law constant of each of the cation sourceand anion source in a standard state was 1.0×10⁴ (Pa/mol·fraction) orgreater, the permeation flow rate, leakage rate, and quantity ofremaining solute were all good. When the Henry's law constant of each ofthe cation source and anion source was 1.00×10⁵ (Pa/mol·fraction) orgreater and the molecular weight of the cation source fell within arange of 45 to 74, these effects were found to be particularly good.When the Henry's law constant of each of the cation source and anionsource was 3.00×10⁵ (Pa/mol·fraction) or greater, these effects werefound to be even better. When the pKb of the cation source fell within arange of 4.0 to 4.5, these effects were found to be still better. Whentrimethyl amine or dimethyl ethyl amine was employed as the cationsource and carbon dioxide was employed as the anion source, theseeffects were found to be particularly good.

Overall energy simulations were conducted for the FO apparatus shown inFIG. 2 for Embodiments 2, 6, and 7 and Comparative Example 3. FIG. 5shows a schematic of the system employed in the simulation. Therein,Absorber denotes the dissolving means, Stripper denotes the separatingmeans, and MIX1 denotes the diluting element. The cation source employedwas ammonia or various amines, and the anion source was carbon dioxide.Data stored in OLI Systems were employed for the various physicalproperties of the cation sources and anion sources. In terms of scale,although not directly relating to the results, the flow rate of thewater that could be purified (TR_WATER) was estimated at 1,000 tons/day.

The concentration of the cation source and anion source in the dilutingelement was diluted with 25° C. pure water from 4.2 mol and 3.0 mol perkilogram of water to 1.4 mol and 1.0 mol, respectively. There wasconsidered to be no loss of cation solution or anion solution duringthis process. The diluted draw solution (Solution) was split at Stripperand Absorber by Split1. The split ratio at that time was about 2:1.However, the Stripper side was set high so that the targetedconcentration would be achieved in the Absorber even when water migratedinto the Absorber along with the cation source and anion source from theStripper. A packed stripping column with 30 theoretical segments wasemployed as the Stripper and there was deemed to be no pressure loss.The pressure was set to a pressure (the saturation vapor pressure of thedraw solution at 25° C.) such that the concentration of the cationsource and anion source reached 4.2 mol and 3.0 mol per kilogram at anultimate temperature of 25° C. when the Absorber contained in theTR_WATER was cooled. The temperature at the bottom of the strippercolumn was set so that at that pressure, the concentration of the cationsource contained in water (TR_WATER) obtained from the bottom of theStripper became 60 μmol per kilogram of water. In the simulation, thegas (vent) removed from the top of the Absorber was set. However, thelevel was so small relative to the whole as to be negligible. Thediluted draw solution (ABS_RC) that was split at Split1 was consideredto have been introduced into the Absorber following dissolution of theanion source and cation source at 25° C. in MIX2. The anion source andcation source were replenished at that time in quantities that made upfor the losses due to retention in the TR_WATER.

As shown in FIG. 5, heat was exchanged by heat exchangers (HX1, HX2) intwo spots. The UA values of the heat exchangers were set to 4.0×10⁸ and2.0×10⁹, respectively. There was considered to be no heat loss throughthe piping, pressure loss, or precipitation of salt due to nonuniformstates. There was considered to be no rate controlling of reaction ratessuch as the hydration reaction or the diffusion rate. Mixed phase flowsof gases and liquids were considered to be suitable. Of the resultsobtained, the level of heating at the bottom of the Stripper wasestimated to be the required energy.

Table 4 shows the simulation results. As indicated in Table 4, therequired energy in Embodiments 6 and 7 and in Comparative Example 3 was100 kWh/m³ (the sum total of the thermal energy required for 1 m³ ofwater) or higher. By contrast, this became 67 kWh/m³ under conditions ofconcentrations of 4.2 mol/kg and 3.0 mol/kg, respectively, in thedissolving means when trimethyl amine was employed as the cation sourceand carbon dioxide was employed as the anion source in Embodiment 2.That indicated considerable energy conservation. This was primarilyattributed to suppression of the quantity of needlessly volatilizingwater in the Stripper.

TABLE 4 Draw solution Stripping performance Cation source Chanege anionsource Level of heating of Amount of change Amount of change Strippercolumn Energy Species [mol/kg] Species [mol/kg] [kWh/m³] conservationExample 2 Trimethylamine 4.2→1.4 CO₂ 3.0→1.0 67 ⊚ Example 6 Isopropylamine 4.2→1.4 CO₂ 3.0→1.0 215 ◯ Example 7 Diethyl amine 4.2→1.4 CO₂3.0→1.0 362 ◯ Comparative Ammonia 4.2→1.4 CO₂ 3.0→1.0 117 ◯ Example 3

Table 5 shows the Absorber temperature and Stripper column bottomtemperature in the above simulation. As indicated in Table 5, inComparative Example 3, the Stripper column bottom temperature, that is,the highest heating temperature, was 63° C. By contrast, in Embodiment2, it was estimated to be 51° C., resulting in a temperaturedifferential with the absorber temperature of 26° C. That was extremelylow relative to the temperature differential of 38° C. of ComparativeExample 3. The fact that the difference in the heating temperature andcooling temperature of the apparatus as a whole was small indicates inand of itself that the apparatus could be run with a low quality energysource. Thus, the results obtained for Embodiment 2 indicate extremelygood economic efficiency.

TABLE 5 Stripper Absorber column bottom Temper- temper- temper- atureEnergy Cation ature ature differ- conser- source (Absorber) (Stripper)ential vation Example 2 Trimethyl- 25° C. 51° C. 26° C. ⊚ amine Compar-Ammonia 25° C. 63° C. 38° C. Δ ative Example 3

KEY TO NUMBERS

-   11 Seawater-   12 Draw solution-   13 Semi-permeable membrane-   14 Stripping column-   15 Volatile component of draw solution-   16 Water-   17 Gas absorber-   18 Pressure gauge-   21 Diluting means-   22 Dissolving means-   23 Separating means-   24 Semi-permeable membrane-   25 Gas absorber

What is claimed is:
 1. A forward osmosis process, comprising: bringing afeed solution and a draw solution comprising a dissolved cation sourceand an anion source into contact through a semi-permeable membrane anddiluting the draw solution with a water separated from the feed solutionby the semi-permeable membrane; separating the diluted draw solutioninto the cation source and anion source and into water; and returningthe separated cation source and anion source to, and dissolving theseparated cation source and anion source in, the diluted draw solution;wherein the cation source comprises an amine compound having a molecularweight of 31 or greater in an uncharged state, and each of the anionsource and the cation source has a Henry's law constant of 1.0×10⁴(Pa/mol·fraction) or greater in water in a standard state.
 2. A forwardosmosis process, comprising: bringing a feed solution and a drawsolution comprising a dissolved cation source and an anion source intocontact through a semi-permeable membrane and diluting the draw solutionwith a water separated from the feed solution by the semi-permeablemembrane; separating the diluted draw solution into the cation sourceand anion source and into water; and returning the separated cationsource and anion source to, and dissolving the separated cation sourceand anion source in, the diluted draw solution; wherein the cationsource comprises one or more members selected from the group consistingof trimethyl amine, dimethyl ethyl amine, isopropyl amine, dimethylamine, and diethyl amine; the cation source has a molecular weight of 31or greater in an uncharged state; and each of the anion source and thecation source has a Henry's law constant of 1.0×10⁴ (Pa/mol·fraction) orgreater in water in a standard state.
 3. A forward osmosis process,comprising: bringing a feed solution and a draw solution comprising adissolved cation source and an anion source into contact through asemi-permeable membrane and diluting the draw solution with a waterseparated from the feed solution by the semi-permeable membrane;separating the diluted draw solution into the cation source and anionsource and into water; and returning the separated cation source andanion source to, and dissolving the separated cation source and anionsource in, the diluted draw solution; wherein the cation source has amolecular weight of 31 or greater in an uncharged state, and each of theanion source and the cation source has a Henry's law constant of 1.0×10⁴(Pa/mol·fraction) or greater in water in a standard state; and furtherwherein the cation source comprises trimethyl amine or dimethyl ethylamine, and the anion source comprises carbon dioxide.